The present invention relates to pressure transmitters. More particularly, the present invention relates to an isolator mounting assembly for supporting a pressure sensor and for isolating the pressure sensor and an interior cavity of the transmitter from a process line fluid to be measured.
Isolator mounting assemblies used to isolate a pressure sensor from the fluid to be measured in a process line are known. Typically, the isolating mounting assembly mounts at the base of a transmitter housing in a hole that opens to an interior cavity of the transmitter. The interior cavity protects known circuitry that connects to the pressure sensor in order to obtain a signal proportional to the pressure in the process line. The isolator mounting assembly includes a connector or fitting that is connectable to the process line to receive the process fluid for measurement. The connector has threads which engage corresponding threads of the transmitter housing to secure the connector to the housing. At an end remote from the process line, the connector accepts a machined plug that has a compliant isolator diaphragm which faces the process fluid. The isolator diaphragm isolates a cavity formed in the plug from the process fluid. The cavity is filled with a substantially incompressible fluid such as silicone oil herein referred to as "fill fluid". A passageway formed in the interior of the plug extends away from the cavity and to an end of the plug remote from the isolator diaphragm. A header is welded to the remote end of the plug and the connecter. The header has a recess which forms a sensor cavity with the end of the plug. The header supports the pressure sensor in the sensor cavity. The sensor cavity is also filled with the fill fluid. Since the sensor cavity is in fluidic communication with the isolator cavity through the passageway, the process fluid pressure is transmitted to the pressure sensor via the fill fluid. The pressure sensor provides an output signal proportional to the fill fluid pressure, and hence, also of the process fluid pressure.
If the pressure sensor measures a gauge pressure or a differential pressure, a mounting assembly supports the pressure sensor on the header and has a passageway that is in fluidic communication with one of the pressures. If the thermal expansion coefficient of the support is substantially different than that of the pressure sensor, temperature variations can cause stresses and strains to be produced in the pressure sensor and the mounting assembly giving rise to stress induced failure and measurement error. This error arises because thermal stresses produced in the pressure sensor cause strain in the sensing elements which are indistinguishable from those changes caused by pressure of the process fluid. Selecting a support material that has a coefficient of thermal expansion that matches that of the pressure sensor is a solution known in the art. However, a continuing problem with the prior art is hermetically sealing and affixing the support to the header.
In addition, although the above-described isolator mounting assembly is well suited for many applications, it is generally not suited for high pressure applications where measured process fluid pressure can exceed approximately 4000 psi.